This invention relates to laser scanning systems and more particularly to devices for effecting the sweeping of a laser beam through a predetermined path.
Various laser-based scanning systems have been disclosed in the patent literature and many are commercially available to read bar codes, e.g., the uniform product code, which are imprinted on packaging for products, or on the products themselves.
One type of scanning system is referred to as a counter or "slot scanner". Another type of scanning system is referred to as a hand-held scanner. Slot scanners are generally mounted within a housing under a checkout counter at a supermarket or other retail establishment. The housing typically includes a window at the top through which a laser beam scanning pattern is projected. The scanning pattern typically consists of plural scan lines which are either parallel to one another and/or intersect one another.
The use of a multi-line scan pattern is dictated, at least in part, by the desire to be able to read the bar code omnidirectionally, i.e., read the bar code irrespective of the orientation of the symbol with respect to the scanner. One common way of producing multi-line scanning patterns in slot scanners is by directing a laser beam onto a rotating polygonal mirror. The rotation of each mirror face has the effect of sweeping the beam directed at it through a predetermined arc, thereby creating a linear scan pattern. The swept beam is then directed to various reflecting surfaces, e.g., mirrors, which are oriented at various angles and positions with respect to one another to convert the single line pattern into the multi-line pattern and to project that pattern out of the window into the space above the window. Hence a bar code held above the window and within the pattern can be read irrespective of the orientation of the bar code.
Examples of prior art slot scanners are found in the following U.S. Pat. Nos. 3,902,048 (Fleischer et al), 3,928,759 (Sansone), 3,988,573 (Hayosh), 3,995,166 (Hobart), 4,006,343 (Izura et al), 4,093,865 (Nickl), 4,097,729 (Seligman et al), and 4,713,532 (Knowles).
Hand-held scanners are devices which are small enough and light enough to be held in a user's hand to enable the aiming of a laser beam produced by the device at a bar code. Such scanners typically include a scanning head or housing containing all of the optical and electrical components to produce a laser beam, to project it out of a window in the head, to receive the light reflected off of the symbol, and to convert that reflected light into an electrical signal indicative of the symbol.
Inasmuch as hand held scanners must be small and light weight and since such devices are arranged to be aimed at the symbol to be read, such scanners commonly produce a pattern consisting only of a single line. This characteristic reduces the number and size of the optical and other components necessary to form the scan pattern.
Examples of various hand-held laser scanners have been disclosed in the following patent literature: U.S. Pat. Nos. 4,387,297 (Swartz et al.), 4,409,470 (Shepard et al.), 4,460,120 (Shepard et al.), 4,575,625 (Knowles), 4,607,156 (Koppenaal et al.) and 4,760,248 (Swartz et al.) and in my co-pending U.S. Pat. Application Ser. No. 07/128,299 filed on Dec. 3, 1987 entitled Ultra Compact, Hand-Held Laser Scanner which is assigned to the same assignee as this invention.
In my aforementioned co-pending U.S. Patent Application there is disclosed an extremely compact, ergonomically designed hand-held laser scanner than that of my aforementioned U.S. patent. The device of that application basically comprises a laser tube and associated high voltage power assembly (or in an alternative embodiment, a semi-conductor laser and associated current and power control means), beam folding and sweeping optics, e.g., an oscillating mirror and associated motor sub-assembly, a photo detector and associated amplification and signal processing circuit sub-assembly, a releasable input/output cable connector or plug, an associated input/output cable, and an on/off trigger assembly.
The use of an oscillating mirror and associated motor assembly offers reduced size and weight characteristics in comparison to a rotating polygon and associated motor for effecting beam sweeping action. However, the use of such a construction is not without drawback. For example, if the motor producing the oscillation is either a stepper motor or a galvanometer type motor (using either a taut band or needle bearings), as is commonly the case, energy of the magnetic field must be used to decelerate the scanning motion and then reverse it. Such action results in a sinusoidal variation of the beam position, and hence a sinusoidal variation of the beam speed across the beam path. As will be appreciated by those skilled in the art such variations in beam speed makes the process of discrimination between objects located a substantial distance, e.g., 10 inches or more, from the device relatively difficult. It is thus desirable to provide a motor which is small in size and weight and whose beam speed (i.e., the speed at which the beam is swept) is linear between reversals, with the duration of each reversal is kept to a minimum, e.g., a graph of the motor motion is saw toothed or triangular.